1) Now this shortcut [CTRL+Y] is used for "delete current line" function (IDM_EDIT_DELLINE Command(4197))
Very usefull function for me. Not for you?
2) Menu "Plugins"--"HotKeys..."--[All keys...]:
look for Command(4152) (or IDM_EDIT_REDO) (or CTRL+SHIFT+Z).
Click it by mouse.
After that push YOUR keys [CTRL+Y]
And now push button [Add]
You can see your keys in list.
Now you will able to use [CTRL+Y] AND [CTRL+SHIFT+Z] for REDO

Would it be possible for Ctrl+Y to be redo by default? I believe that almost every other Windows application has this behavior, and it is a shock to new users to have their redo history wiped when they press Ctrl+Y.

qoshinator, Ctrl+Y's function always was "Delete current line"(and Ctrl+K,Y's - to remove block too). REDO shortcut is Ctrl+Shift+Z and there is will no changes in key binding defaults in nearest and far futures. And about "almost every other Windows application" I can begin holy war included many swears and other aggressive expressions directed to its adepts.